The present invention relates to a telescopic rod antenna and a method for manufacturing the same and, more particularly, to a telescopic rod antenna having a waterproof means for preventing rainwater or the like from soaking into an antenna element through a gap in a joint between a smaller-diameter rod and a larger-diameter rod.
As illustrated in FIG. 14, a prior art telescopic rod antenna generally includes an antenna element 80 constituted by slidably coupling a plurality of rods 81, 82 and 83 of conductive tubes formed of, e.g., brass and having different diameters (three rods are used in this prior art case). The antenna element 80 is insertably held in an antenna element holding tube 90. A certain waterproof means is provided at a joint or a coupling portion X between rods of the antenna element 80.
FIG. 15 is a partly cutaway enlarged sectional view showing the coupling portion X of FIG. 14. In FIG. 15, for example, Ra indicates a smaller-diameter rod corresponding to the rod 81 and Rb represents a larger-diameter rod corresponding to the rod 82.
Referring to FIG. 15, a ring-shaped recess is formed by cutting the inner surface of the larger-diameter rod Rb at the coupling portion X. First and second collars 101 and 102 each shaped like a ring are fitted into the recess, and an O-shaped ring 103 formed of highly elastic member or the like is interposed between the collars 101 and 102. An opening of the tip portion of the larger-diameter rod Rb is constituted as an end-narrowed portion 104 which is narrowed axially by the narrowing process.
In the telescopic rod antenna having the above constitution, as indicated by the broken lines in FIG. 15, water W such as rainwater will soak into the antenna element 80 through a gap between the outer surface of the smaller-diameter rod Ra and the end-narrowed portion 104 at the tip of the larger-diameter rod Rb. Since, however, the water W is blocked with the O-shaped ring 103 which is mounted fluid-tightly between the outer surface of the smaller-diameter rod Ra and the inner surface of the larger-diameter rod Rb, it does not soak below the O-shaped ring 103.
In an antenna element the rods of which are formed of stainless steel, the above-described cutting process is hardly performed since the stainless steel is hard and the rods are formed relatively thin, and usually an O-shaped ring is supported only by narrowing the tip portion of each rod.
FIG. 16 is a partly cutaway enlarged sectional view of an example of a coupling portion X of the stainless steel rods described above. In this example, a ring-shaped swelling portion 105, which is swelled outward by the narrowing process, is formed on the inner wall of the open end portion of the larger-diameter rod Rb, and an O-shaped ring 103 is held in a recess formed in the inner surface of the ring-shaped swelling portion 105.
The foregoing prior art telescopic rod antenna has the following drawbacks.
In the brass rod antenna as shown in FIG. 15, since the inner surface of the larger-diameter rod Rb has to be cut, the manufacturing steps are increased and so are the manufacturing costs. The fluid-tightness between the smaller- and larger-diameter rods Ra and Rb is lost in a relatively short time by variations in the inside and outside diameters of the rods Ra and Rb, wear due to sliding between the O-shaped ring 103 and the smaller-diameter rod Ra, and the like. Therefore, a waterproof function cannot be maintained satisfactorily for a long period of time.
In the stainless steel rod antenna as shown in FIG. 16, since the material of the rods is hard, there occurs no decrease in fluid-tightness due to wear, unlike in the brass rod antenna shown in FIG. 15. In this antenna, the inner surface of the larger-diameter rod Rb need not be cut but the ring-shaped swelling portion 105 has to be formed and the O-shaped ring 103 is fitted into the recess formed in the inner surface of the ring-shaped swelling portion 105. Consequently, there is no great difference in number of manufacturing steps between the rod antennas shown in FIGS. 15 and 16, and the manufacturing cost are increased inevitably.
As described above, conventionally, a relatively expensive O-shaped ring, which is formed of a highly elastic member or the like, is required as a waterproof element to be provided at a joint or a coupling portion X between rods, and any special processing is applied to the rods in order to stably fit the O-shaped ring into a recess. Since there is a case where the waterproof function is not fulfilled completely only by the O-shaped ring 103, grease for sealing or the like is used to make the waterproof function perfect, resulting in increasing in manufacturing steps and manufacturing costs.
Of the rods constituting the antenna element 80, the smallest-diameter rod 81 is provided with a so-called top portion 85 at the tip portion thereof. More specifically, the top portion 85 is attached to the rod 81 in such a manner that a columnar portion 85a projected from the lower end of the top portion 85 is compressed into a hollow of the tip portion of the rod 81 and then the outer circumference of the tip portion of the rod 81 is caulked with rings. In FIG. 14, reference numeral 86 indicates a ring caulking portion. The ring caulking causes some irregularities or distortion on the outer surface of the smallest-diameter rod 81. If the irregularities or distortion is relatively great, the rod 81 is not held smoothly into the subsequent rod 82, and a gap due to the irregularities or distortion occurs between the outer surface of the rod 81 and the inner surface of the rod 82 when the antenna element is retracted. It is thus likely that rainwater or the like will soak into the antenna element through the gap.